This invention relates to a system for sensing the phase sequence of the phase currents supplied to a phase-sensitive, three-phase load to make certain that the load is correctly connected to the three-phase AC power source. While the invention may be employed in any three-phase electrical system where it is desired to monitor the phase sequence, it is particularly attractive when incorporated in the control system (especially a digital logic control system) for a three-phase submersible pump motor used downhole in an oil well, and will be described in that environment.
In pumping fluid from an oil well, one conventional practice is to position a submersible three-phase electric motor and pump assembly in the well bore hole near the bottom thereof. A three-phase AC power supply of appropriate voltage (for example, 1500 volts per phase) couples to and drives the pump motor, which may have a rating of well over 200 horsepower. The average depth of such an oil well is usually around 9,000 feet. Since the pump motor is phase sensitive, it is very important that it be correctly connected to the power supply so that the three phase currents supplied to the motor have the correct phase sequence. Otherwise, the motor will rotate in the wrong direction and the equipment driven by the motor will operate in reverse. As a result, pumping capacity would be reduced and the threaded joints in the pump and interconnecting tubing would tend to unscrew and become disassembled. Hence, it is highly desirable to provide a phase sequence detector for insuring that the pump motor automatically shuts down in the event that a wrong phase sequence occurs.
The present invention provides a novel phase sequence monitoring system for indicating when the three phase currents are incorrectly sequenced and for shutting down the pump motor in the presence of such incorrect sequencing. This is achieved by means of a unique digital logic arrangement which is of relatively inexpensive construction; requires very little power; is highly efficient, accurate and reliable; and can be contained within a very small space.